linux-kernel - RE: [tip:tracing/hw-branch-tracing] tracing/hw-branch-tracing: convert bts-tracer mutex to a spinlock

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Date:	Wed, 25 Feb 2009 09:51:25 +0000
From:	"Metzger, Markus T" <markus.t.metzger@...el.com>
To:	"hpa@...or.com" <hpa@...or.com>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"Metzger, Markus T" <markus.t.metzger@...el.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-tip-commits@...r.kernel.org" 
	<linux-tip-commits@...r.kernel.org>
Subject: RE: [tip:tracing/hw-branch-tracing] tracing/hw-branch-tracing:
 convert bts-tracer mutex to a spinlock

>-----Original Message-----
>From: Ingo Molnar [mailto:mingo@...e.hu]
>Sent: Wednesday, February 25, 2009 9:21 AM

>bts_hotcpu_handler() is called with irqs disabled, so using mutex_lock()
>is a no-no.
>
>All the BTS codepaths here are atomic (they do not schedule), so using
>a spinlock is the right solution.

I introduced the lock to protect against a race between bts_trace_start/stop()
and bts_hotcpu_handler().

Assuming that the hw-branch-tracer is removed and at the same time a cpu
comes online, we might be left with a disabled tracer but still trace
that new cpu.

I wonder whether a simple get/put_online_cpus() would suffice, i.e.

static void bts_trace_start(struct trace_array *tr)
{
	get_online_cpus();

 	on_each_cpu(bts_trace_start_cpu, NULL, 1);
 	trace_hw_branches_enabled = 1;

	put_online_cpus();
}

> static void trace_bts_prepare(struct trace_iterator *iter)
> {
>-	mutex_lock(&bts_tracer_mutex);
>+	spin_lock(&bts_tracer_lock);
>
> 	on_each_cpu(trace_bts_cpu, iter->tr, 1);
>
>-	mutex_unlock(&bts_tracer_mutex);
>+	spin_unlock(&bts_tracer_lock);
> }

Whereas start/stop are relatively fast, the above operation is rather
expensive. Would it make sense to use schedule_on_each_cpu() instead
of on_each_cpu()?

regards,
markus.

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